I have used Pennzoil Syncromesh (Same as GM Syncromesh) In all my DSM trannys. I thought the shifting was vastly improved. You're gonna hear a lot of different opinions about the tranny fluid, just know that all of the fluids listed will work a lot better than just a normal one. It's just up to you to find one YOU like. I'd say try the syncromesh; it's the cheapest out of them all ($5/qt). You can get it at Autozone.
Another way to prolong your tranny is to shift nice and slow. Definately not good if you're trying to race someone, but for normal driving just shift nice and slow, almost pause inbetween gears. And don't downshift to engine brake... just use your brakes
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